1. Field of the Invention
The present invention relates to a circuit board, a semiconductor device, and a method of manufacturing a semiconductor device. More particularly, the invention relates to a circuit board having an electrode to which a solder ball composed of a lead-free solder (a Pd-free solder) is to be connected, a semiconductor device including an electrode and a solder ball which is composed of a lead-free solder and disposed on the electrode, and a method of manufacturing the semiconductor device.
2. Description of the Related Art
With the reduction in size, increase in density, and increase in functionality of electronic equipment in recent years, there have been demands for reduction in size and thickness of electronic components. Under these circumstances, ball grid array (BGA) surface-mount semiconductor device packages have been proposed, in which the mounting area is decreased by the reduction in size, allowing high-density mounting, and which can cope with an increase in the number of input-output pins due to the increase in functionality.
In a BGA semiconductor device, a plurality of spherical, projecting electrodes for external connection, which are also referred to as “solder bumps”, are arranged in a grid pattern on a lower surface of a support board (package board), on an upper surface of which semiconductor elements are placed and fixed. The support board is mounted on a wiring circuit board (motherboard) through the solder bumps, and the electrodes of the support board are connected to wiring portions of the wiring circuit board through the solder bumps.
Examples of a known method of connecting a circuit board of a BGA semiconductor device to a wiring circuit board through solder bumps will be described below with reference to FIGS. 1A, 1B, 1C, and 2. In an example shown in these figures each are an enlarged view of a connecting portion between a circuit board and a wiring circuit board through a solder bump, and semiconductor elements, etc. mounted on an upper surface of the circuit board are not shown.
For performing the method, first, as shown in FIG. 1A, a support board (package board) 10 is prepared, where the support board includes a base composed of an insulating resin, such as a glass epoxy resin, and a plurality of wiring substrates disposed on the base, each wiring substrate having a conductive layer composed of copper (Cu) or the like selectively arranged on the surface thereof. An electrode portion 4 is formed on a principal surface of the support board 10, opposite to a principal surface on which semiconductor elements (not shown) are mounted. The electrode portion 4 includes a copper (Cu) layer 1 containing copper (Cu) as a main component formed with photolithography, or the like, on the base, a nickel (Ni) layer 2 containing nickel (Ni) as a main component, and a gold (Au) layer 3 containing gold (Au) as a main component disposing by plating in that order on the copper (Cu) layer 1. The nickel (Ni) layer 2 and the gold (Au) layer 3 stacked on the copper (Cu) layer 1 can prevent oxidation of the copper contained in the copper (Cu) layer 1.
Then, as shown in FIG. 1B, a solder bump 5, which is a spherical, projecting electrode for external connection, is disposed on the electrode portion 4, the solder bump 5 being composed of a eutectic solder containing tin and lead at a ratio of about 6:4. Then, by performing a heat treatment at a temperature that is equal to or higher than the melting point of the solder bump 5, e.g., 200° C. until the support board 10 is bonded to a wiring circuit board (motherboard) 25, which is described later, via the solder bumps 5.
As described above, since the gold (Au) layer 3 containing gold (Au) as a main component is formed on the top of the electrode portion 4 as shown in FIG. 1A, the solder bump 5 is allowed to spread on the surface of the electrode portion 4, namely on the top layer of the electrode portion 4.
Since disposing the solder bump 5 on the electrode 4 results in diffusion of the gold composing of the gold (Au) layer 1 into the solder bump 5, the surface of the nickel (Ni) layer 2 containing nickel (Ni) as a main component is wetted by the solder bump 5. As a result, a barrier layer 6 composed of tin (Sn) and nickel (Ni) is formed at the junction interface between the nickel (Ni) layer 2 and the solder bump 5.
Next, as shown in FIG. 1C, the solder bump 5 connected to the support board 10 and the electrode 26 is aligned each other, where the electrode 26 is formed of cupper (Cu) on the principal surface of the wiring circuit board 25 composed of a base, such as an insulating resin, on which electro-conductive layers made of copper (Cu) or the like are selectively disposed. Then the solder bump disposed on the support board 10 is bonded to the electrode 26 on disposed on the wiring circuit board 25.
When the support board 10 and the wiring circuit board 25 are bonded to each other through the solder bump 5, as indicated by arrows in FIG. 2, the copper constituting the electrode 26 diffuses into the solder bump 5, and a ternary compound layer 7 composed of tin (Sn)-copper (Cu)-nickel (Ni) is formed at the junction interface between the barrier layer 6 composed of tin (Sn)-nickel (Ni) and the solder bump 5, the ternary compound layer 7 being brittler than the barrier layer 6. That is, on the principal surface of the support board 10, the nickel (Ni) layer 2 containing nickel as a main component, the barrier layer composed of tin (Sn)-nickel (Ni), and the ternary compound layer 7 composed of tin (Sn)-copper (Cu)-nickel (Ni) is formed on the copper (Cu) layer 1 containing copper as a main component, and thereby the layers 1, 2, 6, and 7 form a multilayer structure of that order. Since the solder bump 5 is bonded to the copper (Cu) layer 1 and the nickel (Ni) layer 2 through the barrier layer 6 composed of tin (Sn)-nickel (Ni), sufficient bonding strength can be ensured.
In recent years, in view of environmental protection, it has been required to use a lead (Pb)-free solder, which is free from a lead (Pb) component, for solder bumps.
FIGS. 3A, 3B, 3C, and 4 show an example of a known method of connecting a circuit board of a BGA semiconductor device to a wiring circuit board through solder bumps composed of a lead (Pb)-free solder. These figures show magnified portions of a circuit board and a wiring circuit board which are connected through a solder bump using a lead-free (Pb free) solder. A semiconductors or the like mounded on the circuit board are not shown in these figures. The same components as those in FIG. 1A to 2 are represented by the same reference numerals, and a description thereof will be omitted.
In the method shown in FIG. 3A, first, a support board (a package board) 10 is prepared. Then, a copper (Cu) layer 1 containing copper (Cu) as a main component, a nickel (Ni) layer 2 containing nickel (Ni) as a main component, and a gold (Au) layer 3 containing gold (Au) as a main component are disposed in that order on a principal surface of a support board 10, opposite to a principal surface on which semiconductor elements (not shown) are mounted. The copper (Cu) layer 1 is formed by using the photolithography, and the nickel (Ni) layer 2 and the gold (Au) layer 3 are formed by using the electroless plating. The copper layer 1, the nickel layer 2, and the gold layer 3 constitute an electrode portion 4. The nickel (Ni) layer 2 and the gold (Au) layer 3 are formed on the copper (Cu) layer 1 can prevent oxidation of the copper contained in the copper (Cu) layer 1.
As shown in FIG. 3B, a solder bump 15, which is a spherical, projecting electrode for external connection, is disposed on the electrode portion 4, the solder bump 15 being composed of a lead (Pb)-free solder containing tin (Sn)-gold (Ag)-copper (Cu). Then, by performing a heat treatment at a temperature that is equal to or higher than the melting point of the solder bump 15, e.g., 230° C. or higher until the support board 10 is bonded to a wiring circuit board (motherboard) 25, which is described later, via the solder bumps 15.
As described above, since the gold (Au) layer 3 containing gold (Au) as a main component is formed on the top of the electrode portion 4 as shown in FIG. 3A, the solder bump 15 is allowed to spread on the surface of the electrode portion 4, namely on the top layer of the electrode portion 4.
When the solder bump 15 is disposed on the electrode portion 4, the gold (Au) constituting the gold (Au) layer 3 diffuses into the solder bump 15, and the solder bump 15 wets the surface of the nickel (Ni) layer 2 containing nickel (Ni) as a main component. As a result, a ternary compound layer 7 composed of tin (Sn)-nickel (Ni)-copper (Cu) is formed at the junction interface between the nickel (Ni) layer 2 and the solder bump 15.
Referring to FIG. 3C, the solder bump 15 bonded to the support board 10 is aligned with an electrode 26 composed of copper (Cu) disposed on a principal surface of a wiring circuit board (motherboard) 25. Then, by performing a heat treatment at a temperature that is equal to or higher than the melting point of the solder bump 15, e.g., 230° C. or higher, the solder bump 15 bonded to the support board 10 is bonded to the electrode 26 composed of copper (Cu) disposed on the principal surface of the wiring circuit board 25.
Thereby, as indicated by arrows in FIG. 4, the copper (Cu) constituting the electrode 26 of the wiring circuit board 25 diffuses into the solder bump 15, and the ternary compound layer 7 composed of tin (Sn)-copper (Cu)-nickel (Ni) which is brittler than the barrier layer 6 shown in FIGS. 1B, 1C, and 2 is allowed to grow. That is, on the principal surface the support board 10, the nickel (Ni) layer 2 containing nickel as a main component is formed on the copper (Cu) layer 1 containing copper as a main component. A multilayer structure in which the ternary compound layer 7 composed of tin (Sn)-copper (Cu)-nickel (Ni) is disposed on the nickel (Ni) layer 2 is formed.
Furthermore, International Publication No. 01/076335 pamphlet proposes an embodiment in which an electronic component is mounted on a pad of a substrate through a connection layer containing a solder, and a diffusion prevention layer is disposed in the connection layer, the diffusion prevention layer preventing copper (Cu) in a base layer of the pad from diffusing into the solder of the connection layer.
Japanese Laid-open Patent Publication No. 2006-179798 proposes an embodiment in which a composite layer including a nickel (Ni) layer and a palladium (Pd) layer is disposed on a pad of a substrate, a solder is provided on the composite layer, and by performing a reflow treatment, a structure including a nickel (Ni) layer, a nickel (Ni)-tin (Sn) alloy layer, and a solder bump is formed.
However, in the case of the example using the lead-free (Pb free) solder bump 15 composed of the lead-free solder shown in FIGS. 3a to 4, the ternary compound layer 7 composed of tin (Sn)-copper (Cu)-nickel (Ni) is formed at a junction interface between the nickel (Ni) layer 2 and the solder bump 15. The ternary compound layer 7 is brittler than the barrier layer 6 composed of tin (Sn)-nickel (Ni) formed at the junction interface between the nickel (Ni) layer 2 and the solder bump 15 in the known example using the solder bump 5 composed of the lead (Pb) shown in FIGS. 1A to 2.
Consequently, the bonding strength between the solder bump 15 and the nickel (Ni) layer 2 provided with the ternary compound layer 7 composed of tin (Sn)-copper (Cu)-nickel (Ni) on the surface thereof in the embodiment shown in FIGS. 3A to 4 is lower than the bonding strength between the solder bump 5 and the nickel (Ni) layer 2 provided with the barrier layer 6 composed of tin (Sn)-nickel (Ni) on the surface thereof in the embodiment shown in FIGS. 1A to 2.
As described above, the heat treatment is performed at a temperature higher than the melting point of the solder bump 15 disposed on the electrode 4, such as 230° C. or higher, until the support board 10 and the wiring circuit board 25 are bonded each other. Thus there is a possibility that dimensional changes may occur in different planar directions between the support board 10 and the wiring circuit board 25 according to a temperature difference between the temperature higher than the melting point and the room temperature after the heat treatment.
In such a case, the stress generated at the junction interface between the nickel (Ni) layer 2 and the solder bump 15 increases, and the adhesion strength between the nickel (Ni) layer 2 and the solder bump 15 decreases. As a result, there is a possibility that, for example, the solder bump 15 may be separated, resulting in disconnection, thus degrading mounting reliability.
In particular, in the case where electroless plating containing phosphorus (P) is used for forming the nickel (Ni) layer 2 on the copper layer 1, a phosphorus-rich layer is formed on the surface of the nickel (Ni) layer 2, and there is a possibility that the bonding strength between the nickel (Ni) layer 2 and the solder bump 15 may be decreased compared with the case where a known solder containing lead (Pb), such as a tin (Sn)-lead (Pb) solder, is used.